The present invention relates generally to sphere illumination and more particularly to an improved method of and improved assembly for measuring equivalent sphere illumination.
The term "equivalent sphere illumination", commonly referred to and hereinafter referred to as ESI, is well known in the lighting industry. The illumination on a task, for example, the writing on a piece of paper, within a room depends upon the position of the task within the room and the particular lighting arrangement utilized therein. In any event, while most conventional lighting arrangements do not provide complete sphere illumination, they do provide illumination, which produces some visibility equal to the visibility produced by an equivalent amount of sphere illumination. The term ESI refers to this level of sphere illumination.
The amount of ESI on a task within a conventionally lit room will depend not only on the position of the task within the room but also upon the orientation of the task relative to the point at which it is viewed. If, for example, at the viewing point of the task there is a loss of contrast from veiling reflection, i.e., reflected glare off the task, the ESI as viewed at that point will be small. On the other hand, where there is little loss in contrast at the viewing point, the ESI will be greater.
An often used "yardstick" of the lighting industry in evaluating the quality of illumination in a lighted room is the amount of ESI on a task, as viewed from a particular point. A typical task is a typed letter or other dark symbol on a sheet of white paper, the sheet being located on a flat, horizontal surface. A standard viewing point is above the paper at an angle of approximately 25.degree. from a line normal to the paper, i.e., the average viewing point of a person sitting at a desk or table reading what is typed on the paper. Generally, as stated above, it has been found that less veiling reflection from the paper results in higher ESI values while greater reflected glare results in lower ESI values.
While ESI is one sound and, more than likely, permanent tool in evaluating given lighting arrangements, there has heretofore been a substantial drawback resulting from its use. This drawback resides in the manner in which the ESI is measured. Most of the known methods provided heretofore are complicated, time consuming and expensive. The drawback just referred to has been overcome by a method of and assembly for measuring ESI disclosed in U.S. application Ser. No. 446,247 (Griffith et al.), filed Feb. 27, 1974, and assigned to the Assignee of the present application, now U.S. Pat. No. 3,912,399.
As will be seen hereinafter, the present invention is directed to improvements in the method and assembly disclosed in this application. More specifically, the method and assembly disclosed in the application just referred to utilizes two preferably identical tasks, one of which is the task being evaluated located in the lighted area being evaluated and the other of which is a control task located in a light controlled area. Applicants of thee present invention have discovered several factors which will affect the accuracy of the ESI measurement taken by means of the method and assembly disclosed in the Griffith et al. application.
First, Applicants have discovered that it is not only preferably to use "identical" tasks as stated in the Griffith et al application but to maximize accuracy it is essential to use identical tasks. Second, Applicants have discovered that two independent tasks may be provided and may be made to appear identical to the naked eye, for example two pieces of paper with the same word typed thereon. However, Applicants have found that for purposes of determining ESI using the Griffith et al. method and assembly, these tasks are not identical and, even though they may be as identical as can be achieved by known duplication methods and may appear identical to the naked eye, the unobserved differences can adversely affect the accuracy of the ESI value obtained. Applicants have discovered this by first taking an ESI reading using one task as the task being evaluated and a second identically appearing task as the control task and then repeating this procedure but reversing these tasks, everything else remaining the same. The two readings, in several tests of this type were not always the same. In fact, Applicants have found as large as a 30% difference in these readings where, if the tasks were clearly identical, the reading should be the same.